Ikeda Masato, Longnecker Richard
Department of Microbiology-Immunology, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA.
Virology. 2007 Apr 10;360(2):461-8. doi: 10.1016/j.virol.2006.10.046. Epub 2006 Dec 5.
Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) plays a key role in regulating viral latency and EBV pathogenesis by functionally mimicking signals induced by the B cell receptor (BCR) altering normal B cell development. LMP2A specifically associates with Nedd4 family ubiquitin-protein ligases which downmodulate LMP2A activity by ubiquitinating LMP2A and LMP2A-associated protein tyrosine kinases (PTKs). Since specific ubiquitin tags provide an endocytic sorting signal for plasma membrane proteins which traffic to membrane vesicles, we examined LMP2A localization and trafficking. We found that LMP2A is secreted through exosomes, small endocytic membrane vesicles, as previously demonstrated for LMP1. Interestingly, the treatment of cells with methyl-beta-cyclodextrin (MCD), which depletes cholesterol from plasma membrane, dramatically increased LMP2A abundance and LMP2A exosome secretion. Cholesterol depletion also blocked LMP2A endocytosis resulting in the accumulation of LMP2A on plasma membrane. LMP2A phosphorylation and ubiquitination were blocked by cholesterol depletion. LMP2A in the exosomal fraction was ubiquitinated but not phosphorylated. These results indicate that cholesterol-dependent LMP2A trafficking determines the fate of LMP2A degradation.
爱泼斯坦-巴尔病毒（EBV）的潜伏膜蛋白2A（LMP2A）通过功能性模拟B细胞受体（BCR）诱导的信号来改变正常B细胞发育，从而在调节病毒潜伏和EBV发病机制中发挥关键作用。LMP2A与Nedd4家族泛素蛋白连接酶特异性结合，该连接酶通过使LMP2A和LMP2A相关的蛋白酪氨酸激酶（PTK）泛素化来下调LMP2A活性。由于特定的泛素标签为转运到膜泡的质膜蛋白提供了内吞分选信号，我们研究了LMP2A的定位和转运。我们发现LMP2A通过外泌体（小的内吞膜泡）分泌，正如之前对LMP1的证明。有趣的是，用甲基-β-环糊精（MCD）处理细胞，其可从质膜中耗尽胆固醇，这显著增加了LMP2A的丰度和LMP2A外泌体分泌。胆固醇耗尽还阻断了LMP2A的内吞作用，导致LMP2A在质膜上积累。胆固醇耗尽可阻断LMP2A的磷酸化和泛素化。外泌体部分中的LMP2A被泛素化但未被磷酸化。这些结果表明，依赖胆固醇的LMP2A转运决定了LMP2A降解的命运。
